In this year, we conducted four sub-projects. First, we employed oxygen sensors coupled with high-speed amperometry to evaluate fluctuations in brain oxygen levels induced by morphine, an opioid analgesic drug frequently abused by humans. Second, we employed this technology to examine changes in brain oxygen induced by cocaine and the role of peripheral neural substrates in mediating these effects. Third, we examined how benzodiazepine drugs affect hypoxic effects of intravenous heroin. Finally, we examined the role of peripheral and central opioid receptors in mediating brain hypoxia induced by heroin.